(a) United States of America
Mass production was an important feature of the US economy in the 1920s, with Henry Ford pioneering the assembly line method for car manufacturing in Detroit.
Source: Chapter 3, Section 3.3 – Rise of Mass Production and Consumption
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The passage explicitly states: "One important feature of the US economy of the 1920s was mass production." Henry Ford's assembly line at Detroit is the key example. For MCQs, just selecting the correct option with a one-line justification is sufficient. No other country listed is mentioned in connection with mass production as a defining feature.